yeah I'll second that - split relays are the way to go.
much easier on the battery as it is always being topped up, plus the added convenience of not having to put your battery on to che=arge at night, plus as its always charged you'll not lose a days work due to battery being flat.
just remember to check fuse once a week/and, or test that baattery is charging with a multimeter every now and then. Keep spare fuses in the van as they can blow once in a while and you don't want to be caught short over a 15p fuse. But even if it does fuse and the battery goes flat, you can replace fuse and continue working, they charge really fast so even 10mins with the engine running will give you around 30mins of pump running full speed.
we have a split relay in each van and have never topped up batteries or changed them in over 5 years.